1. Technical Field
The present invention relates to a piezoelectric resonator element to be housed in a package or case, a piezoelectric device in which the piezoelectric resonator element is housed in a package or a case, and a gyro sensor.
2. Related Art
Piezoelectric devices, including a piezoelectric resonator and a piezoelectric oscillator housing a piezoelectric resonator element in a package, have been widely used for small information equipment, such as hard disc drives (HDD), mobile computers and integrated circuit (IC) cards, for mobile communications equipment, such as cellular phones, car phones and paging systems, and for measuring instruments such as gyro sensors.
FIG. 13 is a plan view schematically showing a publicly known structure example of a piezoelectric resonator element that is used for such a piezoelectric device. JP-A-2002-76806 is an example of related art.
Referring to the drawing, this piezoelectric resonator element 1 is made of single-crystal quartz, for example, and includes a base 2 having an extra width and two resonating arms 3, 4 extending in parallel with each other from the base 2 in one direction. FIG. 14 is a sectional view along line A-A of FIG. 13. Long grooves 5, 6 are provided to the front and back surfaces of the resonating arms 3, 4, respectively, along their longitudinal direction. To the long grooves 5, 6, an excitation electrode (not shown) is provided as a driving electrode.
A driving voltage applied to the excitation electrode from outside effectively produces an electric field in the resonating arms 3, 4. Consequently, the resonating arms 3, 4 perform flexural vibration in a way that their end parts move closer to and away from each other as shown in FIG. 13. A vibration frequency based on this vibration is output to be used for a reference signal, such as a controlling clock signal.
The resonating arms 3, 4 included in the piezoelectric resonator element 1 and the long grooves 5, 6 of the resonating arms 3, 4 are formed by etching a substrate made of a piezoelectric wafer material. In general, a wafer substrate is etched to provide the outer shape of a tuning fork as shown in FIG. 13. Then the long grooves 5, 6 as shown in FIG. 14 are formed by half etching.
This piezoelectric resonator element 1, however, involves the following problem. In wet etching for forming the outer shape, etching progresses at different rates in the electrical axis X, the mechanical axis Y and the optical axis Z shown in FIGS. 13 and 14. Due to this etching anisotropy, the long grooves 5, 6 are not provided with a flat base.
As a result, the right and left side walls of the resonating arm 3, for example, that sandwich the long groove 5 are different in thickness. Therefore, the side walls of the resonating arm 3 on the right and left of the virtual central line CE shown in FIG. 13 have different hardness. Specifically, the left side wall is harder than the right side wall.
Under the state where the resonating arms 3, 4 are performing flexural vibration as shown in FIG. 13, the degree of flexure deformation different between right and left in the respective horizontal directions. Consequently, flexure of the right and left resonating arms 3, 4 is out of balance. As a result, stresses F1 and F2 transmitted to the base 2 because of the deformation of the resonating arms 3, 4 are not equal and do not negate each other, thereby causing Z-axial or Y-axial displacement. Consequently, crystal impedance (CI) is considered to increase.